Presentation Description

No description available.

Comments

Posting comment...

Premium member

Presentation Transcript

CAPNOGRAPHY:

DR. DEBABRATA SAHA CAPNOGRAPHY

HISTORY & BACKGROUND :

HISTORY & BACKGROUND Capnography , the measurement of CO2 in respiratory gases has become an integral part of anaesthesia monitoring. Luft developed the principle of capnography in 1943 from the knowledge that CO2 is one of the gases that absorbs infra-red (IR) radiation of a particular wavelength. The accuracy of rapid IR CO2 analysis in determining alveolar carbon dioxide concentration was established by Collier and his colleagues . The value of the end tidal sample established by Ramwell . In 1978 Holland was the first country to adopt capnography as a standard of monitoring during anaesthesia.

Co2 Physiology :

Co2 Physiology Carbon dioxide (CO2) is a waste product of normal cellular metabolism. CO2 leaves the cells and is carried by the venous blood to the heart (circulation) and lungs (respiration). Once CO2 reaches the lungs, it is eliminated in the process of exhalation. In order for CO2 to be effectively eliminated from the body, there must be adequate blood flow to the lungs, adequate gas exchange across the alveolar-capillary membrane, and adequate ventilation of the lungs to “blow off” the CO2. Therefore, changes in respired CO2 may reflect alterations in metabolism, circulation, respiration, the airway or breathing system.

INTRODUCTION :

INTRODUCTION Capnography is useful for much more than checking the position of the endotracheal tube. It provides information about CO2 production, pulmonary perfusion, alveolar ventilation, respiratory patterns and elimination of CO2 from the anaesthesia circuit and ventilator . Thus it gives us a rapid and reliable method to detect life threatening conditions such as malposition of tracheal tubes, ventilatory failure, circulatory failure and defective breathing circuits. The American Society of Anaesthesiologists (ASA) study showed that 34% of damaging events involve the respiratory system and proposed that 93% of these cases could have been prevented by monitoring withpulse oximetry , capnography or both. For this reason the ASA included capnography in its standards for basic monitoring.

TERMINOLOGY :

TERMINOLOGY Capnography is the graphic display of instantaneous CO2 concentration versus time (Time Capnogram ) or expired volume (Volume Capnogram ) during a respiratory cycle. Capnograph is the machine that generates a waveform and the capnogram is the actual waveform. Capnometry is the measurement and numerical display of maximum inspiratory and expiratory CO2 concentrations during a respiratory cycle. Capnometer is the device that performs the measurement and displays the reading. However, the capnograph or capnogram , is vastly preferable to a meter or even a fast digital display.

CHEMICAL METHOD OF CO2MEASUREMENT:

CHEMICAL METHOD OF CO2MEASUREMENT A pH-sensitive chemical indicator is enclosed in a plastic housing and is connected to the gas stream between the endotracheal tube and the anesthesia circuit. The pH sensitive indicator changes color when exposed to C0 2 .1 The color varies between expiration and inspiration, as C0 2 level increases or decreases. The color changes from purple (when exposed to room air or oxygen) to yellow (when exposed to 4% C0 2 ). The response time of the device is sufficiently fast to detect changes of C0 2 breath-by breath.

PHYSICS :

Infra Red Spectrography PHYSICS Infra-red spectrographs has been the most popular technique for monitoring C0 2 . The wavelength of IR rays exceeds 1.0 milli micron while the visible spectrum is between 0.4 and 0.8 milli microns. The IR rays are absorbed by polyatomic gases (non-elementary gases such as nitrous oxide (N20), C0 2 , and water vapour. Carbon dioxide selectively absorbs specific wavelengths (4.3 milli microns) of IR light.

Cont… :

Since the amount of light absorbed is proportional to the concentration of the absorbing molecules , the concentration of a gas can be determined by comparing the measured absorbance with the absorbance of a known standard. The C0 2 concentration measured by the monitor is usually expressed as partial pressure in mmHg Cont…

Cont… :

Cont… Molecular Correlation Spectrography (MCS) The source, unlike the black body IR radiator, is an all glass discharge lamp, without an electrode , coupled with an IR transmitting window . The glass lamp undergoes a special cleaning process as well as chemical conditioning before being filled with a carefully balanced mixture of up to 7 gases at low pressure.

Cont… :

MCS Electrons generated by a radio frequency voltage excite Nitrogen molecules, one of the gases within the source. Carbon dioxide molecules are then excited through collision with the excited nitrogen molecules as energy is transferred from Nitrogen to the CO 2 molecules. As the excited CO 2 molecules drop back to their ground state they emit signature wavelength of CO 2 , which is the radiation emitted from the source Cont…

Cont… :

Mass Spectrography Raman Spectrography Raman Spectrography uses the principle of " Raman Scattering " for CO 2 measurement. The gas sample is aspirated into an analyzing chamber, where the sample is illuminated by a high intensity monochromatic argon laser beam. The light is absorbed by molecules which are then excited to unstable vibrational or rotational energy states (Raman scattering). The mass spectrograph separates molecules on the basis of mass to charge ratios. Cont…

TYPES OF CAPNOGRAPHS :

Cont… :

Cont… :

Side-stream Capnographs Sensor is located in the main unit and CO 2 is aspirated via a sampling tube (6ft) connected to a T-piece adapter located between endotracheal tube and breathing circuit. Cont…

SIDE-STREAM CAPNOGRAPHS:

Disadvantages 1.Easy to connect 2.No problems with sterilization 3.Can be used in awake patients 4.Easy to use when patient is in unusual positions such as in prone position 5.Can be used in collaboration with simultaneous oxygen administration via a nasal prong 1.Delay in recording due to movement of gases from the ET to the unit 2.Sampling tube obstruction 3.Water vapor pressure changes affect CO 2 concentrations 4.Pressure drop along the sampling tube affects CO 2 measurements 5.Deformity of capnograms in children due to dispersion of gases in sampling tubes SIDE-STREAM CAPNOGRAPHS Advantages

MAIN- STREAM CAPNOGRAPHS:

Advantages Disadvantages 1.No sampling tube 2. No obstruction 3.No affect due to pressure drop 4. No affect due to changes in water vapor pressure 5.No pollution 6 .No deformity of capnograms due to non dispersion of gases 7.No delay in recording 8. Suitable for neonates and children 1.Long electrical cord. 2. Facial burns have been reported with earlier versions . 3.Sensor windows may clog with secretion. 4 .Difficult to use in unusual patient positioning such as in prone positions. MAIN- STREAM CAPNOGRAPHS

PHYSIOLOGY OF CAPNOGRAPHY:

PHYSIOLOGY OF CAPNOGRAPHY Dead space refers to the ventilated areas which donot participate in gas exchange . Total or physiologic dead space refers to the combination of the following spaces- 1.Anatomical dead space- airways leading to alveoli These areas are not associated with pulmonary perfusion and therefore do not participate in gas exchange. 2. Alveolar dead space- refers to ventilated areas which are designed for gas exchange i.e.alveoli , but do not actually participate.This can be caused by lack of perfusion.Eg - embolism 3. Mechanical dead space- refers to external artificial airways which add to total dead space,as when a patient being mechanically ventilated.Mechanical dead space is an extension of anatomic dead space.

Cont… :

Cont… :

HOW EVOLVE ?? :

HOW EVOLVE ?? At the end of inspiration , assuming that there is no rebreathing , the airway and the lungs are filled with CO 2 -free gases.

Cont… :

Cont… Carbon dioxide diffuses into the alveoli and equilibrates with the end-alveolar capillary blood (PACO 2 = PcCO 2 = 40 mm Hg). The actual concentration of CO 2 in the alveoli is determined by the extent of ventilation and perfusion into the alveoli (V/Q ratio).

Cont… :

Cont… The alveoli with higher ventilation in relation to perfusion (high V/Q alveoli) have lower CO 2 compared to alveoli with low V/Q ratio that would have higher CO 2 . As one moves proximally in the respiratory tract, the concentration of CO 2 decreases gradually to zero at some point. The volume of CO 2 -free gas is termed respiratory dead space

Cont… :

Cont… here there is no exchange of oxygen (O 2 ) and CO 2 between the inspired gases and the blood. As the patient exhales, a CO 2 sensor at the mouth will detect no CO 2 as the initial gas sampled will be the CO 2 -free gas from the dead space.

Cont… :

Cont… As exhalation continues, CO 2 concentration rises gradually and reaches a peak as the CO 2 rich gases from the alveoli make their way to the CO 2 sensing point at the mouth.

Cont… :

Cont… At the end of exhalation, the CO 2 concentration decreases to zero (base line) as the patient commences inhalation of CO 2 free gases

Cont… :

Cont… The evolution of CO 2 from the alveoli to the mouth during exhalation, and inhalation of CO 2 free gases during inspiration gives the characteristic shape to the CO 2 curve which is identical in all humans with healthy lungs

Cont… :

FAST CAPNOGRAM :

TREND CAPNOGRAM :

TREND CAPNOGRAM The overall CO 2 changes (trend) can be followed at a slow (about 0.7 mm.sec-1) speed .

PowerPoint Presentation:

Normal features of a capnogram . A : Baseline, represents the beginning of expiration and should start at zero. B : The transitional part of the curve represents mixing of dead space and alveolar gas . C : The alpha angle represents the change to alveolar gas . D :The alveolar part of the curve represents the plateau average alveolar gas concentration . E : The end-tidal carbon dioxide value. F : The beta angle represents the change to the inspiratory part of the cycle. G : The inspiration part of the curve shows a rapid decrease in carbon dioxide concentration. Components of a time capnogram

Cont… :

Lower part of the lung is better ventilated Lower part of the lung is also better perfused Cont…

PowerPoint Presentation:

Lower part of lung is relatively more perfused than ventilated, resulting in spectrum of alveoli with higher V/Q ratio (lower CO 2 ) on the top of the lung, and lower V//Q (higher CO 2 ) at the bottom of the lung Cont…

Cont… :

Cont… Expiratory segment. The expiratory segment of a time capnogram is divided into three phases: I, II, III . Occasionally, at the end of phase III, a terminal upswing, phase IV, may occur. Phase I Represents the CO 2 -free gas from the airways (anatomical and apparatus dead space ). Phase II Consists of a rapid S-shaped upswing on the tracing (due to mixing of dead space gas with alveolar gas). Phase III Consists of an alveolar plateau representing CO 2 -rich gas from the alveoli . It almost always has a positive slope, indicating a rising PCO 2 and is due to the following reasons:

Cont… :

Cont… (I) Steady excretion of CO 2 into the alveoli . Carbon dioxide is being continuously excreted into the alveoli which are becoming progressively smaller as expiration continues. This results in a steady increase in alveolar PCO 2 towards the end of expiration, and hence contributes to a rising positive slope of phase III as expiration proceeds. (ii) The late emptying of alveoli with lower ventilation/perfusion (V/Q) ratios and, therefore, relatively higher PC02. If all the alveoli had the same PC02, then irrespective of the emptying patterns, phase III would be nearly horizontal . However, this ideal situation does not occur, even in normal lungs which have a wide range of V/Q ratios. Some alveoli have a higher V/Q ratio (over ventilated) than ideal alveoli and hence they have a relatively lower PC02. Others have a lower V/Q ratio than ideal alveoli (under ventilated) resulting in a relatively higher PC02. The delayed emptying of these alveoli with low V/Q (high PC02) contributes to the rising slope of phase III.

Cont… :

Cont… There may be a regional variation in ventilation per unit perfusion producing a spectrum of V/Q ratios ( spatial mismatching). Under these circumstances, the slope of phase III is determined by the nature of emptying of the alveolar units: synchronous or asynchronous. If the units empty synchronously, the gas from well- perfused and underperfused alveoli is expired simultaneously, resulting in a horizontal phase 111 or else a phase 111 with minimal slope . However, if the units empty asynchronously, units with longer time constants, hence higher PC02, would empty later (sequential emptying) resulting in a rising slope of Phase III. The slope of the phase III is dependent, therefore, on the emptying patterns of various alveoli with different V/Q ratios as well as continuous C02 excretion into the alveoli. The relative contributions of all of the above mechanisms cannot be separated and all occur simultaneously, influencing the height or the slope of phase III. Factors, such as changes in cardiac output, CO 2 production, airway resistance and may further affect the V/Q status of the various units in the lung, and thus influence the height or the slope of phase III. This attribute makes capnography a useful diagnostic tool to detect abnormalities in ventilation perfusion mismatch of the lung.

Cont… :

Cont… Inspiratory segment - Phase 0 After phase III is complete, the descending limb makes an almost right angle turn and rapidly descends to the base line. This represents the inspiratory phase during which the fresh gases (CO 2 -free gases) are inhaled and CO 2 concentration falls rapidly to zero. The segment of the CO 2 trace from the beginning of inspiration to the beginning of expiration, which includes the descending limb and the initial part of the horizontal base line, can be designated as phase 0 . The later part of the horizontal base line is the phase I of expiratory segment. Phase 0 represents dynamics of inspiration.

Cont… :

Cont… The Alpha angle The angle between phases II and III, has been referred to as the alpha angle. Increases as the slope of phase III increases . Normally it is between 100 and 110. The alpha angle is an indirect indication of V/Q status of the lung. Airway obstruction causes an increased slope and a larger angle. Other factors that affect the angle are the response time of the capnograph , sweep speed, and the respiratory cycle time.

Cont… :

Cont… The Beta angle The nearly 90 degrees angle between phase III and the descending limb in a time capnogram has been termed as the beta angle. This can be used to assess the extent of rebreathing . During rebreathing , there is an increase in beta angle from the normal 90 degrees. Other factors, such as prolonged response time of the capnometer compared to respiratory cycle time of the patient, particularly in children , can produce increase in the beta angle with the elevation of the baseline

Cont… :

Cont… Rebreathing If a patient is rebreathing its own exhaled gas then the CO 2 levels will rise. Inhaled air will not be free of CO 2 so the baseline will start to rise . The PetCO 2 will also rise but it is more apparent from the capnogram than from the numerical value. This can be extremely valuable for ventilated patients or patients connected to a rebreathing anesthetic circuit

VOLUME Vs TIME CAPNOGRAPH:

VOLUME Vs TIME CAPNOGRAPH

VOLUME CAPNOGRAPH :

VOLUME CAPNOGRAPH Carbon dioxide concentration can be plotted against time (time capnogram ) or expired volume ( Volume capnogram / CO 2 expirogram / CO 2 spirogram ) during a respiratory cycle. A volume capnogram has only an expiratory segment . The expiratory segment of a volume capnogram is divided into three phases, phase I, II, and III. Phase III of a volume capnogram is a better representation of V/Q status of lung than the phase III of time capnogram

Cont… :

Cont… Advantages of time capnography : 1. Simple and convenient: 2. Monitor non- intubated patients 3. Monitors dynamics of inspiration as well as expiration : Disadvantages of time capnography : 1. Poor estimation of V/Q status of the lung 2. Can not be used to estimate components of physiological deadspace . Volume capnography has several advantages over time capnography First, the volume of CO2 exhaled per breath can be measured. Second , significant changes in the morphology of the expired wave form can be detected in the volume capnogram (e.g. secondary to PEEP) that are not seen in the traditional time capnogram tages over time capnography . Third, dead space can be partitioned into components of interest.

C0nt… :

C0nt… Monitor non- intubated patients: The CO 2 analyzer used in SBT-CO 2 tracings are mainstream capnometers , where the CO 2 sensor is inserted between the endotracheal tube and breathing circuit. Hence, endotracheal intubation is required for plotting an SBT-CO 2 curve( Single Breath Test). A time capnogram does not require a ventilator and can be used to monitor spontaneous ventilation without breathing through the ventilator. This is because time capnographs make use of side strem sensors . Time capnographs with side-stream sensors have the sensor located in the main unit itself; the sample of gas is aspirated from the patient’s airway, via a tiny pump, through a 6-ft capillary tube, into the unit. This enables time capnography to monitor non- intubated patients, as the sampling of respired gases is obtained from the nasal cavity using nasal adapters.

Cont… :

Cont… Poor estimation of V/Q status of the lung- The V/Q status of the lung is more accurately reflected in the slope of phase III in an SBT-CO 2 trace In a time capnogram , the gradient of the phase III slope is usually less obvious . This may be because a smaller volume of expired gases (approximately the final 15%) occupies half the time available for expiration, so that a similar change in CO 2 is distributed over a greater length of time in the time capnogram than in the SBT-CO 2 trace.

Capnogram and Tidal vol :

Capnogram and Tidal vol A horizontal line (red line in the figure) representing PaC02 (arterial blood sampled during the PETCO 2 recordings) is drawn on the CO 2 trace. The area under the curve, green area, is the volume of C02 in the breath and represents effective alveolar ventilation .

Cont…:

CARDIAC OUTPUT & PETCO2 :

CARDIAC OUTPUT & PETCO2

Cont… :

Cont…

Cont… :

Cont… Cardiac output and (a-ET)PCO 2 Reduction in cardiac output and pulmonary blood flow result in a decrease in PETCO 2 and an increase in (a-ET)PC02. The percent decrease in PETCO 2 directly correlates with the percent decrease in cardiac output. Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO 2 . Consequently alveolar dead space is reduced as is (a-ET)C02. A PETCO 2 greater than 30 mm Hg was invariably associated with a cardiac output more than 4 L/min or a cardiac index > 2 L/min. Furthermore, when PETCO 2 exceeded 34 mm Hg, pulmonary blood flow was more than 5 L/min (CI > 2.5 L).

Phase IV :

Phase IV Slow alveoli empty out more evenly (expiratory flow rate is more constant) and the increase in PCO 2 is spread more evenly over the tidal volume. Fast alveoli empty fast initially and then slow down . Most of the expirate leaves in the first second at almost constant PCO 2 . Towards the end of expiration, there is only a small amount of expired gas flow thereby increasing PCO 2 steeply .

Cont… :

Cont… A terminal upswing is observed at the end of phase III under certain circumstances, such as during anesthesia /IPPV in obese patients and in pregnant subjects. The initial part of phase III is horizontal, or has a minimal slope, and lies below the level of PaCO 2 . The terminal part of phase III ascends steeply and may reach PaCO 2 . The terminal upswing, or rise, is known as phase IV because it resembles phase IV in the SBT-N2 curve

(a-ET)CO2 difference & alv dead space:

(a-ET)CO2 difference & alv dead space

Cont… :

Cont… Normal values of (a-ET)PCO 2 is 2-5 mm Hg. (a-ET)PCO 2 as an index of alveolar dead space There is a positive relationship between alveolar dead space and (a-ET)PCO 2 . (a-ET)PCO 2 increases with age, emphysema, and in circumstances where alveolar dead space increases such as in low cardiac output states, hypovolemia , and pulmonary embolism . (a-ET)PCO 2 decreases in pregnancy and children.

NEGATIVE (a-ET)CO2 DIFFERENCE ??:

NEGATIVE (a-ET)CO2 DIFFERENCE ?? Where can they occur? 1.Healthy subjects during low frequency high tidal volume ventilation 2.Pregnant subjects 3.Infants and Children 4.After coming off cardiac bypass 5.During and after exercise What are the reasons for negative values? 1.Experimental errors 2.Rebreathing 3.Inadvertent addition of CO 2 to the inspired gases 4.Physiological reasons

CLINICAL ASPECTS :

Analysis of capnographs :

Analysis of capnographs Five characteristics of capnogram should be evaluated 1.Frequency 2.Rhythm 3.Height 4.Baseline 5.Shape

Cont… :

Dual waveform capnogram This capnogram occurs due to sampling leaks in the sampling tube cracks, or a loose connection between sampling tubes and the monitor . This shape of a capnogram can result in differential lung emptying of the lungs . For example one may be a normal lung (transplanted lung) and other, a diseased lung. The normal lung has greater carbon dioxide to expire than the diseased lung. In addition, a dual capnogram can result during endobronchial intubation and in patients with severe kyphoscoliosis . Cont…

Cont… :

Signature capnogram Capnogram recorded during the use of Bain anesthetic system / Mapelson D. The base line is elevated from zero. During inspiration, there a small rebreathing wave due to inhalation of carbon dioxide. Red indicates inspiration. A similar capnogram has been reported during closed circuit anesthesia and IPPV where soda lime was totally exhausted Cont…

Cont… :

Ripple effect This capnogram was recorded during low frequency controlled ventilation. Cardiogenic oscillations shown during the expiratory plateau and the descending limb waveform occurring as a result of movement of gases in the airway due to cardiac pulsations. Cont…

Cont… :

Cont… Slanting and prolongation of expiratory upstroke indicative of obstruction to gas flow caused by a partially obstructed tracheal tube or obstruction in the patient’s airways (chronic obstructive lung disease or bronchospasm ).

Cont… :

Cont… :

Trend capnogram during cardiac arrest / resuscitation. Cont…

Cont… :

Rebreathing Trend showing gradual elevation of baseline Cont…

Cont… :

General anesthesia for cesarean section The slope of the phase III is increased. This is a normal physiological variation. Airway obstruction can result in an increase in the phase III as well. Depending on the severity of airway obstruction, phase II can also be prolonged. Occasionally, a phase IV can also occur in capnograms recorded during anesthesia in pregnant subjects . Cont…

Cont… :

Curare cleft Cont… Patients who are partially paralysed with muscle relaxants may make respiratory efforts. This can alert the anaesthesia provider that the muscle relaxant is wearing off. It may occur anywhere in the respiratory cycle. If the depression occurs in the later third of the waveform it is referred to as curare cleft.

Cont… :

Resembling curare cleft due to an artifact created by surgeon leaning on the chest, or pushing against the diaphragm during expiration . Partial disconnect of main stream capnometer Cont…

Cont… :

Elevation of base line A classic representation of rebreathing . Cont…

Cont… :

A gradual decrease in end tidal carbon dioxide Reduced metabolism, hypothermia, hyperventilation, small tidal volume ventilation due to inadequate alveolar sampling, and leaks in the sampling system, decrease in cardiac output Cont…

Cont… :

Cont… Occasionally, there can be a reverse phase 3 slope seen in patients with emphysema . This may be due to destruction of alveolar capillary system in emphysematous lungs resulting in the delivery of carbon dioxide to expired gases.

Cont… :

Right main stem intubation (EBI) This capnogram probably occurred as the tip of the endotracheal tube changed while moving in and out of the right main bronchu s during the respiratory cycle. This resulted in a partial obstruction to gases from the left lung, thereby prolonging expiratory time from the left lung. The initial peak is due to the carbon dioxide from the well-ventilated right lung. The second peak is most likely due to the prolonged expiratory time of the poorly-ventilated left lung . Cont…

Cont… :

Cont… Therefore, capnography is a non-sensitive diagnostic tool to detect EBI. PETCO2 can increase, decrease, or remain unaltered, depending on the circumstances. A normal PETCO2 and normal capnogram will not rule out EBI. On the other hand, an increased PETCO2, reduced PETCO2, or an abnormal capnogram should encourage one to consider using EBI in the differential diagnosis of hypoxia.

Cont… :

Cont… :

Cont… :

Cont… Air leak due to a broken connection between sampling tube and capnograph

Cont… :

Cont… Lung transplant - Dual capnogram

Cont… :

Cont… Increased CO 2 due to hypoventilation, hypermetabolic states and rebreathing

Cont… :

Cont… Capnogram during spontaneous ventilation in adults

Cont… :

Cont… These capnograms can occur in children and neonates . Variations are normal and due to faster respiratory rates, smaller tidal volumes, relatively longer response time of the capnographs . Recent technological advances such as micro-stream analyzers are reducing the artifacts due to faster response time and thereby producing normal looking capnograms even in children and neonate.

Cont… :

Cont… Sampling problems such air or oxygen dilution during nasal or mask sampling of carbon dioxide in spontaneously breathing patients.

Cont… :

Cont… Slit sampling tube can result in a pig tail capnogram . However, during spontaneous ventilation no such deformity observed, although end-tidal CO2 is lower than the normal.

Cont… :

Cont… A terminal upswing at the end of phase 3 , known as phase 4 , can occur in pregnant subjects, obese subjects .

Cont… :

Cont… The capnogram shows prolonged phase II as well as slanting of descending limb of inspiratory segement . The expiratory unidirectional valve is noted to be fixed in the open position and thus allowing previously exhaled gases, high in CO2, to be rebreathed with each inspiration. The CO2 waveform returned to normal soon after the valve is removed, cleaned, dried, and replaced .

Cont… :

Cont… Plateau is prolonged due to rebreathing An incompetent inspiratory valve will allow exhaled CO2- containing gas to enter the inspiratory limb of the circuit during expiration. During the next inspiration, the CO2-containing gas in the inspiratory limb enters the patient, extending the expiratory alveolar plateau (phase III) of the time capnogram . A decrease in CO2 follows the extended alveolar plateau and represents the appearance of the CO2-free gas from the machine end of the inspiratory limb. During this latter phase of inhalation, CO2 concentration may reach zero.

Cont… :

Cont… Occasionally enough alveolar gas can be forced down the esophagus into the stomach during mask ventilation resulting transiently in few capnograms as shown . The concentration of carbon dioxide falls dramatically after three breaths and to zero in about six breaths. However, a careful analysis of capnograms should alert to such a possibility of esophageal intubation particularly if the tip of the endotracheal tube was not seen going past the vocal cords. If enough alveolar gas is forced down the esophagus into the stomach during mask ventilation prior to intubation, the concentration of carbon dioxide in the later may be similar to that in the alveoli. After few breaths, the carbon dioxide in the stomach will then be diluted and the waveform will very quickly become flat. Esophageal intubation following a mask ventilation

Cont… :

Cont… :

Cont… Carbonated beverages in the stomach can result in abnormal capnograms with progressively decreasing CO 2 values following esophageal intubation.

Cont… :

Cont… Contamination of capnometer results in the sudden elevation of base line as well as ETCO 2 values.

Capnography and laparoscopy :

Capnography and laparoscopy Capnography has three important applications during laparoscopic surgery . 1. It serves as a non-invasive monitor of PaCO 2 during CO 2 insufflation and therefore can be used to adjust ventilation. 2. It may help in the detection of accidental intravascular CO 2 insufflation (HR,MBP,CVP,PIP). 3. It may help in the detection of complications of CO 2 insufflation such as pneumothorax , and hemorrhage .

Pitfalls of Time Capnography :

Pitfalls of Time Capnography Time capnography is a convenient method, commonly used in anesthetic practice to monitor ventilation. There are, however, subtle but important limitations of this process. 1. The V/Q status of the lung is more accurately reflected in the slope of phase III in an SBT CO 2 trace (volume capnogram ) than in that of a time capnogram , in which the gradient of phase III slope is usually less obvious. This may be because a smaller volume of expired gases (approximately the final 15%) occupies half the time available for expiration, so that a similar change in CO 2 is distributed over a greater length of time in the time capnogram than the SBT-CO 2 trace. 2. Unlike an SBT-CO 2 trace, the physiological dead space and CO 2 output cannot be determined from a time capnogram as currently recorded . 3. Presently, the available time capnographs do not distinguish the end of expiration from the beginning of inspiration. This drawback makes the analysis of time capnograms during inspiration difficult, and results in an inaccurate assessment of rebreathing , which is grossly underestimated .

Capnography tips :

Capnography tips Discipline is the key to success. Let capnography play its role in helping you to reach a diagnosis as quickly as possible. You can derive maximum benefit from the science of capnography if you follow some of these guidelines in day to day anesthesia practice.

Cont… :

Cont… Calibration of the monitor Maintain and calibrate the monitor as per manufacturer's guidelines . For accurate measurements capnographs should be calibrated first, zeroing the monitor to room air, and then administering a gas of known CO 2 concentration. As the changes in barometric pressure affect the PETCO 2 measurements, calibration procedures should be performed using the same type of sampling tube as will be used when the analyzer is connected to the patient sampling. Omission of standard narrow 2 m long sampling tube (at the time of calibration) will result in the absence of a large pressure drop across the ends of the tube, and a measuring error does becomes inevitable during subsequent clinical use.

Cont… :

Cont… 1 .Before any interpretations are made of PETCO 2 readings and waveforms one should ascertain that the capnograph is functioning correctly. 2 . Contamination of the unit with secretions or mucus may cause distortion of CO 2 waveforms. 3 . Similarly partial obstruction of sampling lines with water can result in distorted waveforms. A leak in the sampling tube may result in low PETCO 2 measurements. Check the monitor to see if is functioning properly. 4.A simple, but less accurate, method is to record a normal CO 2 tracing ( eg ., one's own). The typical CO 2 waveform with PETCO 2 readings between 38-42 mm Hg, confirms the proper functioning of the capnograph . Checking the monitor

Cont… :

Cont… Low level of the monitor may facilitate condensed water and secretions move towards the analyzer thereby resulting in blockade of filters Monitor above the level of the patient may facilitate condensed water and secretions not go up the sampling tube

Cont… :

Cont… Position the sampling vertically upwards Use water filters at both ends of sampling tube Water droplets, patient secretions and coalescence of water aerosols from humidifiers in breathing circuits may result in an accumulation of water and secretions in the breathing hoses . The contaminants enters the sampling tubes and increases flow resistance in the tubing thus affecting accuracy of the CO 2 measurements. Some times, liquids can enter the main unit of the analyzer despite the presence of water traps where they can cause corrosion and form residues. Positioning the sampling tube upwards away from the patient decreases the frequency with which liquids are drawn into the tubes . Interposing filters at either ends of the sampling tube can also minimize the contamination of the CO 2 monitor.

Capnograph of the millennium :

Capnograph of the millennium A vision of an ideal capnograph A side stream capnograph with the advantages of main stream capnograph seems to be the ideal one . Inspiratory and expiratory flow rates should be displayed above the co 2 waveforms to delineate segments and phases of capnogram . A capability of plotting a volume capnogram would enable determination of physiological deadspace . Prevention of secretions being drawn into the capnographs would markedly improve the ICU application capnographs .

:

SUMMARY

PowerPoint Presentation:

Normal features of a capnogram . A : Baseline, represents the beginning of expiration and should start at zero. B : The transitional part of the curve represents mixing of dead space and alveolar gas . C : The alpha angle represents the change to alveolar gas . D :The alveolar part of the curve represents the plateau average alveolar gas concentration . E : The end-tidal carbon dioxide value. F : The beta angle represents the change to the inspiratory part of the cycle. G : The inspiration part of the curve shows a rapid decrease in carbon dioxide concentration. Components of a time capnogram

Cont… :

Cont… The Alpha angle The angle between phases II and III, has been referred to as the alpha angle. Increases as the slope of phase III increases . Normally it is between 100 and 110. The alpha angle is an indirect indication of V/Q status of the lung. Airway obstruction causes an increased slope and a larger angle. Other factors that affect the angle are the response time of the capnograph , sweep speed, and the respiratory cycle time.

Cont… :

Cont… The Beta angle The nearly 90 degrees angle between phase III and the descending limb in a time capnogram has been termed as the beta angle. This can be used to assess the extent of rebreathing . During rebreathing , there is an increase in beta angle from the normal 90 degrees. Other factors, such as prolonged response time of the capnometer compared to respiratory cycle time of the patient, particularly in children , can produce increase in the beta angle with the elevation of the baseline

VOLUME Vs TIME CAPNOGRAPH:

VOLUME Vs TIME CAPNOGRAPH

VOLUME CAPNOGRAPH :

VOLUME CAPNOGRAPH Carbon dioxide concentration can be plotted against time (time capnogram ) or expired volume ( Volume capnogram / CO 2 expirogram / CO 2 spirogram ) during a respiratory cycle. A volume capnogram has only an expiratory segment . The expiratory segment of a volume capnogram is divided into three phases, phase I, II, and III. Phase III of a volume capnogram is a better representation of V/Q status of lung than the phase III of time capnogram

Cont…:

Cont… :

Cont… Cardiac output and (a-ET)PCO 2 Reduction in cardiac output and pulmonary blood flow result in a decrease in PETCO 2 and an increase in (a-ET)PC02. The percent decrease in PETCO 2 directly correlates with the percent decrease in cardiac output. Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO 2 . Consequently alveolar dead space is reduced as is (a-ET)C02. A PETCO 2 greater than 30 mm Hg was invariably associated with a cardiac output more than 4 L/min or a cardiac index > 2 L/min. Furthermore, when PETCO 2 exceeded 34 mm Hg, pulmonary blood flow was more than 5 L/min (CI > 2.5 L).

Cont… :

Cont… Normal values of (a-ET)PCO 2 is 2-5 mm Hg. (a-ET)PCO 2 as an index of alveolar dead space There is a positive relationship between alveolar dead space and (a-ET)PCO 2 . (a-ET)PCO 2 increases with age, emphysema, and in circumstances where alveolar dead space increases such as in low cardiac output states, hypovolemia , and pulmonary embolism . (a-ET)PCO 2 decreases in pregnancy and children.